Modern electronic systems, for example, mobile phones, digital cameras and personal digital assistants, have an ever increasing demand for highly-integrated and energy-efficient semiconductor circuits. In order to meet these requirements, the physical size of field effect transistors (FETs) within the semiconductor circuits is reduced. Furthermore, the voltage supply is reduced to save power as well as to meet the requirements of the reduction in the physical size of the FETs and to avoid an increase of electrical fields of scaled FETs.
Due to the shrinking size and the reduction of the voltage, the FETs become more susceptible to degradation effects, like Negative Bias Temperature Instability (NBTI), HCI (Hot Carrier Instability) and Positive Bias Temperature Instability (PBTI). These degradation effects occur over the lifetime of the semiconductor circuits and are accelerated by high temperatures, high voltages, and a high number of switching events in digital circuits. NBTI causes a shift of a threshold voltage (VT) of a p-channel field effect transistor (PFET) and similarly, PBTI causes a shift of VT of an n-channel field effect transistor (nFET). HCI affects the mobility of carries in FETs and is caused by switching events in the digital circuits, when input and output nodes of a logic gate are changing their logic states.
In order to avoid malfunction of a semiconductor circuit, degradation of the semiconductor circuit needs to be monitored. Malfunctions that may occur due to degradation include a reduction of the switching speed of digital circuits, a relative delay shift of synchronous signals within a semiconductor circuit, such as clock signals at different parts of the semiconductor circuit, or a complete non-functionality of pipelined circuits due to hold time variations.